The following relevant prior art literature is being referred to:
German Offenlegungsschrift No. 1,401,466 PA1 German Offenlegungsschrift No. 1,751,325 PA1 British Patent No. 687,896 PA1 British Patent No. 1,109,395 PA1 Nuclear Energy, January 1961, page 13. PA1 English, R. E., und R. N. Weitmann: Experience in investigation of components of alkali-metal-vapor space power systems. "Alkali metal coolants", IAEA, Vienna 1966, P.711-725. PA1 Rossbach, R. J. und G. M. Kaplan: Potassium testing of condensate removal devices for Rankine space power turbines. 6th IECEC (1971) No. 719059. PA1 Bond, J. A., und M. U. Gutstein: Component and overall performance of an advanced Rankine cycle test rig. 6th IECEC (1971) No. 719064. PA1 Frass, A. P.: A potassium-steam binary vapor cycle for better fuel economy and reduced thermal pollution. ASME-Winter Annual Meeting, Washington, December 1971, 71-WA/Ener-9. PA1 Wilson, A. J.: Space power spinoff can add + 10 points of efficiency to fossil-fueled power plants. 7th IECEC (1972) No. 729050. PA1 Niggemann, R. G. 3000 hour endurance test of a 6 kWe organic Rankine cycle power system. 7th IECEC (1972). No. 729053.
In the construction of large thermal power plants the cost factor in respect of fuel, heat transfer and conversion and antipollution considerations for exhaust air and waste water has gained ever increasing importance. A solution of these problems is primarily contingent on and rendered possible by increasing the efficiency of the thermal power plant. An increase in the efficiency of the plant results at the same time in a lowering of the specific plant investment costs for all those structural elements which are primarily dependent on the thermal efficiency, for example the plant buildings, the heating surfaces, the condensers, the cooling water supply, the transport of fuel and the like. For this reason an increase in the efficiency of the plant is desirable from an economical point of view.
However, a substantial increase in the efficiency or output of a thermal power plant is only feasible from a practical point of view by using additional circuits, particularly alkali metal vapor circuits of high temperature which in certain instances may be combined with further circuits as, for example, a gas turbine circuit or process. For this reason it has recently and repeatedly been proposed to operate thermal power plants with such additional circuits.
In thermal power plants of the nature with which this invention is concerned, the heat which is generated as a result of a nuclear process or by combustion of fossil fuels, is utilized for the vaporization of alkali metals, particularly potassium. The potassium vapor which is thus formed is utilized in a potassium-steam turbine for recovery of mechanical energy and is subsequently condensed.
With a view to obtaining satisfactory total efficiency, the heat of the potassium energy conversion process has to be utilized for the recovery of further amounts of energy. Customarily this is achieved by utilizing the condensation heat of the potassium vapor for the vaporization of water into steam or for the superheating of steam. The steam thus formed is then subsequently utilized in customary manner in a steam turbine for energy recovery purposes and finally its condensation heat, in the form of lost heat, is discharged to the ambient atmosphere from a condenser. From a practical point of view, the vapor condensation stage of the alkali metal circuit and the steam generating stage are arranged in heat transfer relationship in a heat exchanger.
In respect to all those alkali metals which, from a practical point of view, can be considered for such procedures -- potassium being the customary alkali metal for this purpose -- the specific steam volume at temperatures below 450.degree. C. is already so large that economical use in temperature ranges below 450.degree. C. is hardly feasible. The lower process temperature of a potassium-steam circuit system is therefore customarily about 450.degree. C. or above.
If the potassium energy conversion circuit or process is coupled with a water (steam) energy conversion procedure in the above indicated manner, considerable difficulties are encountered inasmuch as the heat of the potassium vapor can be transferred to the steam loop under exceptionally large energy losses only. This is so because the major portion -- particularly the portion that is attributed to the vaporization procedure proper -- of the enthalpy increase in the steam circuit takes place at a level which is substantially below the condensation temperature of the potassium vapor. Further, in case of a defect in the potassium/water heat exchanger, water or steam of high pressure would enter the potassium vapor condensation space. This in turn, due to the pressure differential and also due to vigorous chemical reaction, would lead to very substantial damages and could even result in complete destruction of the heat exchanger. The occurrence of such a defect becomes the more likely the higher the pressure in the steam circuit, since the walls of the potassium/water heat exchanger, whose thickness of course increases with increasing pressure, are then subjected to higher thermal stresses, particularly during the starting and closing down stages so that an increased possibility for operational disturbances can be expected. On the other hand, the energy loss referred to above at the time of the heat transfer can only be decreased if extremely high pressures prevail in the steam circuit.
The construction of the potassium-water heat exchanger thus constitutes a major problem to avoid reaction between water and potassium in case of defect.
Further, as stated, high system pressures in the steam circuit cannot be avoided in view of the substantial differentials between potassium vapor condensation temperature and steam generating temperature. This problem cannot successfully be overcome by superheating of the steam.